| Tombusviridae | |
|---|---|
| Virus classification | |
| Group: | Group IV ((+)ssRNA) |
| Family: | Tombusviridae |
| Genera | |
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Aureusvirus |
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Tombusviridae is a family of single-stranded positive sense RNA plant viruses. The name is derived from the type species of the Tombusvirus genus, Tomato bushy stunt virus (TBSV). Viruses in this family are classified as Type IV viruses in the Baltimore classification, and are part of the luteovirus supergroup.[1]
The RNA is encapsulated in an icosahedral (T=3) capsid, composed of 180 units of a single coat protein 27-42K in size; the virion measures 28-35 nm in diameter, and it is not enveloped.[2]
Viruses in this family are primarily soil-borne, some transmitted by fungal species of the order Chytridiales, others by no known vector. Virions may spread by water, root growth into infected soil, contact between plants, pollen, or seed, depending on the virus species. These viruses may be successfully transmitted by grafting or mechanical inoculation, and both the virion and the genetic material alone are infective.[2]
Contents |
All Tombusviridae have a single linear genome, with the exception of Dianthoviruses, whose genome is in two parts.[3] The genome is approximately 4.6-4.8kb in length, with a 5' cap, and it encodes 4-6 ORFs. The polymerase encodes an amber stop codon which is the site of a readthrough event within ORF1, producing two products necessary for replication. There is no helicase encoded by the virus.
Members of Tombusviridae replicate in the cytoplasm, by use of negative strand templates. The replication process leaves a surplus of positive sense (+)RNA strands, and it is thought that not only does the viral RNA act as a template for replication, but is also able to manipulate and regulate RNA synthesis.
The level of RNA synthesis has been shown to be affected by the cis-acting properties of certain elements on the RNA (such as RNA1 and 2[4][5]), which include core promoter sequences which regulate the site of initiation for the complementary RNA strand synthesis. This mechanism is thought to be recognised by RNA-dependent RNA polymerase, found encoded within the genome.
Tombusviridae have been found to co-opt GAPDH, a host metabolic enzyme, for use in the replication center. GAPDH may bind to the (-)RNA strand and keep it in the replicase complex, allowing (+)RNA strands synthesized from it to be exported and accumulate in the host cell. Downregulation of GAPDH reduced viral RNA accumulation, and eliminated the surplus of (+)RNA copies.[6]
Research has shown that infection of plants from tombusviruses contain defective interfering RNAs that are born directly from the viruses RNA genome, and no host genome. Viral DI RNAs with their small size and cis-acting elements are good templates both in vivo and in vitro on which to study RNA replication.
Sub-genomic RNA is used in the synthesis of some proteins; they are generated by premature termination of (-)strand synthesis. sgRNAs and sgRNA negative-sense templates are found in infected cells.[2]
The family Tombusviridae includes the following genera: